Recently, there has been developed a system which takes radiographic images of an object by using a large-area semiconductor image sensor. This system has the practical merit of being capable of recording images over an extremely wide radiation exposure range as compared with a conventional radiographic image system which uses silver halide radiography.
That is, the semiconductor image sensor reads X-rays in an extremely wide dynamic range as electrical signals by using a photoelectric conversion means, and converts the electrical signals into digital signals. These digital signals are processed to output radiographic images as visible images to recording materials such as photosensitive materials and display devices such as CRTs, thereby obtaining good radiographic images even if radiation exposure dose varies to some extent.
In radiography using this semiconductor image sensor, kymography which radiographs the lungs along with respiration in the lung field portions is expected to provide new pathological information in place of conventional pathological diagnoses mainly using still images. A radiographic image processing apparatus which can obtain dynamic chest images suitable for diagnosis is disclosed in Japanese Patent Laid-Open No. 2003-290193. In this case, “radiographs the lungs along with respiration” means to radiograph the lungs from a sufficiently expanded state to a sufficiently contracted state so as to obtain moving images; radiography preferably includes one cycle from an expansion phase to a contraction phase of the lungs.
With regard to front chest images radiographed in medical examination and the like, differences between two images radiographed at a time interval are calculated. The effect of obtaining differences over time is that the manner in which a morbid portion changes between the two images can be clearly expressed. With this operation, an improvement in diagnosis by a doctor can be expected. Japanese Patent Laid-Open No. 2003-174590 discloses a method of efficiently generating a difference image from two high-resolution still images radiographed at a time interval. In general, however, conventional chest radiography is generally performed while the breath is held; images which have been provided for time-varying difference processing are still chest images radiographed while the breath is held.
As described above, radiography using a semiconductor image sensor can be continuously performed along with respiration. However, there has been known no difference processing technique between radiographic images radiographed during respiration at a time interval.
Realizing difference display of even a plurality of sets of respiratory moving images radiographed at a time interval without making the object hold his/her breath makes it possible to explicitly display a change in disease. This improves diagnosis accuracy. When a morbid portion overlapping a rib occurs at the time of maximum respiration, since the relative positional relationship between the rib and the lung changes during respiration, the morbid portion can be observed at a specific time point during respiration in some cases.
In chest still image radiography, a patient is generally radiographed while he/she inhales most deeply and holds his/her breath in accordance with an instruction from a radiographer. Therefore, a phase in a respiratory cycle is specified. In contrast to this, when chest moving images are to be obtained by radiography along with respiration, it is difficult to apply X-rays upon specifying a phase in a respiratory cycle.
As a method of applying X-rays upon specifying a phase in a respiratory cycle, a sensor which detects respiration may be additionally used. In this case, however, a tool must be attached to the patient, and radiography becomes complicated.
In addition, normal respiration is performed unconsciously, and respiratory cycles are relatively stable which the object is at rest. On the other hand, at the time of respiratory radiography, it is preferable for an object to take a deeper breath than normal in order to improve diagnostic performance. In this case, however, respiratory cycles greatly vary in radiographic operations as compared with the case of unconscious respiration. Therefore, the difference method for chest still images cannot be simply applied to such operation.